If you have configured your e-mail address as a POP account then you will have the option to decide whether downloaded mail should remain on the server as well as how long it should remain there.
The following steps will assist you in adjusting the setting for leaving mail on the server in Apple Mail:
1. Click on the Mail menu and choose Preferences.
2. Click on the Accounts icon and select your e-mail account from the list on the left-hand side.
3. Click on the Advanced button and change the settings of the check box called Remove copy from server after retrieving a message:
- If the check box is unchecked, then all of your messages will always stay on the server.
- If the check box is checked, then the messages will only stay on the server for period specified in the drop-down box below (select this option if you access this account from more than one device).
- If the check box is checked and the drop-down box is set to Immediately, then mail will be downloaded and removed from the server right away.
Select the option which will meet your requirements.
4. Click OK to confirm.
Note: This applies to all versions of Mail with the exception of OS X Sierra.